This invention relates to the manufacture of glyoxylic acid esters by oxidation of the corresponding esters of glycolic acid and, more particularly, to an improved process for recovery of water-free and alkanol-free glyoxylic acid ester from the oxidation reaction mass.
Polyacetal carboxylates have been demonstrated to be useful as builders in detergent formulations. Crutchfield U.S. Pat. No. 4,144,226 describes the preparation of polyacetal carboxylates by polymerization of an ester of glyoxylic acid, preferably methyl glyoxylate. The glyoxylic acid ester monomer may be prepared by vapor phase oxidation of the corresponding ester of glycolic acid. Side reactions occurring under the oxidation reaction conditions result in the contamination of the reaction product with water and with an alkanol derived from the ester. To minimize the loss of yield to side reactions, the oxidation reaction is carried out with a deficiency of oxygen, so that the reaction mixture also contains a substantial fraction of unreacted glycolate ester.
In order to obtain a satisfactory yield and a high quality polyacetal carboxylate product from the polymerization reaction, it is necessary that the glyoxylate monomer be of high purity and that, in particular, it be purified to be substantially free of water, alkanol, and unreacted glycolate. In accordance with the process described in U.S. Pat. No. 4,502,923, the product of the oxidation reaction is subject to multiple distillation operations, first at low temperature under vacuum for removal of low boilers, i.e., water and methanol, then at higher temperature under vacuum for removal of glycolate ester as an overhead stream, and finally at atmospheric pressure for removal of glyoxylate ester as an overhead stream. As indicated by an inflection in the vapor/liquid equilibrium curve, more glycolate ester can be removed from a mixture containing glyoxylate ester at low absolute pressure than at atmospheric pressure. The converse is true for glyoxylate ester. Bottoms from the glyoxylate atmospheric pressure distillation contain the glycolate that has not been removed as overhead in the glycolate vacuum still, as well as the hemiacetal of the glycolate and glyoxylate, and other high boilers. This stream is recycled to an earlier step in the process, typically the feed to the low boiler still.
Glyoxylate ester reacts with water to form the hydrate, and with both alkanol and glycolate to form the corresponding hemiacetals. These are equilibrium reactions which may proceed in either direction not only in the reaction step but also in the distillation steps and beyond. Although the first vacuum distillation step may be effective for removal of free water and alkanol, glyoxylate hydrate and glyoxylate/alkanol hemiacetal remain in the the still bottoms and are carried forward to subsequent steps where they may be decomposed to form additional free water and alkanol. Under the conditions of the atmospheric glyoxylate still, in particular, removal of glyoxylate ester from the liquid phase tends to promote the decomposition of hydrate and alkanol hemiacetal.
Various efforts have been made in the art to produce a glyoxylate monomer substantially free of water and alkanol. In one such process, described in Chou et al. U.S. Pat. No. 4,502,923, methyl glycolate is added in the vacuum distillation of low boilers, thereby converting free methyl glyoxylate to the glycolate/glyoxylate hemiacetal in accordance with that equilibrium reaction. By thus reducing the concentration of free glyoxylate, the addition of glycolate promotes decomposition of the glyoxylate hydrate and methanol hemiacetal in the vacuum distillation, so that the water and methanol they contain can be substantially removed in that step. However, the effectiveness of this procedure is limited by the equilibrium relationships, and quantitative removal of water and alkanol can be achieved only at the expense of dealing with a large fraction of glycolate/glyoxylate hemiacetal in the bottoms product. A high concentration of glycolate in the system reduces the productivity of the process and increases the energy requirements of the separation steps.
Christidis U.S. Pat. No. 4,156,093 describes a process in which hemiacetal esters of glyoxylic acid are produced by reacting aqueous glyoxylic acid with an excess of alkanol, preferably butanol, and simultaneously dehydrating the reaction product by an azeotropic distillation in which the alkanol serves as the azeotroping agent, alkanol being refluxed to the reaction pot. Thereafter, the excess alkanol is distilled off under vacuum, the residue treated with phosphoric acid to convert the hemiacetal ester to the aldo-ester, and the residue redistilled under vacuum to recover the aldo-ester.
Japanese patent No. 57-176,929 describes a process in which glyoxylic acid ester is produced by pyrolizing a hemiacetal in a distillation column in the presence of benzene, the alkanol produced on pyrolysis being removed in the form of a benzene/alcohol azeotrope.
German Offenlegungsschrift No. 33 23 372 describes a process in which a glyoxylic acid ester is produced by oxy-dehydrogenation, apparently of a glycolic acid ester. An entrainer, typically pentane or cyclohexane, is added to the reaction mixture, after which the resulting mixture is fed to the central point of a fractionating column. Water of reaction, other low boilers and entrainer distill off the top of the column, and glyoxylic acid ester is left as a sump product.
Japanese patent No. 171789 describes a process for obtaining glyoxylic acid ester obtained by reacting 1-4 carbon alcohols, in the presence of azeotropic solvents, benzene and dichloroethane. The reaction mixture is distilled to recover the ester.